Litharge is an important article of commerce, used on a large scale in a number of industrial manufacturing processes. In the manufacture of glass, high purity litharge, for example containing less than 0.01% lead, is required.
Molten litharge is extremely corrosive and molten lead is capable of dissolving many metals. Prior to the present invention, no material has been found that could withstand the hostile environment of molten lead and molten litharge at the elevated temperatures encountered in this reaction. Control of the reaction also posed a difficult problem as the oxidation of lead at elevated temperatures is extremely rapid and highly exothermic, like a flame. The temperature in the flame may reach up to 1700.degree. C. All previous attempts to produce molten litharge by oxidation of molten lead have been defeated by this corrosivity, which is destructive of conventional refractory furnace linings. Furthermore, a pure product could not be obtained, due to contamination by components of the failed refractory.
Litharge of sufficiently high purity for use in the glass industry has previously been manufactured by the Barton pot process. In this process, lead is melted and fed to the Barton pot where it is agitated and contacted with air at 450.degree. to 550.degree. C. In the Barton pot, the pool of molten lead is stirred by high speed blades. This throws up droplets which are oxidised by the air, but the oxidation is incomplete. The solid powder product, containing from about 70% to 99% PbO, is entrained in the air stream while the heavier lead droplets fall back into the pool. The powder product is separated by filtering the air stream from the Barton pot, typically using a baghouse. Handling of the dusty powder leads to environmental problems. The powder product is then calcined in a separate reactor, if necessary, to produce a solid product containing at least 99% PbO, which is fed to a melting furnace. In the melting furnace, most of the final traces of lead are oxidised and the product is then granulated to produce litharge of purity of 99.9%+.
The Barton pot process is limited by the requirement for multiple steps, involving an expensive train of equipment, and also by the fact that the maximum size of a Barton pot is limited, which frequently creates the need for a number of Barton pots to achieve a desired production level. The Barton pot process and other prior art processes are described in "LEAD OXIDES--Chemistry--Technology Battery Manufacturing Uses--History" (1974), Independent Battery Manufacturers Association, Inc., Florida USA, at pages 21 to 25. Reference is made to Barton's U.S. Pat. Nos. 988,963 (1911) and 1,060,153 (1913), Pope and Barton U.S. Pat. No. 633,533 (1899), Mayer 2,235,487 (1941), and Vahernkamp et al 3,322,496 (1967).
In describing a "fused litharge furnace" with reference to Hughes U.S. Pat. No. 975,955 (1910) and Petraeus U.S. Pat. No. 592,594 (1897), which is said to be "now mainly of historical interest" this book comments that "A mixed bath of lead and litharge at about 1000.degree. C. has almost fantastic corrosive and erosive properties" which has caused major problems.
The book also describes the "fume type process", which produced a "smoke" from which a product of fine particle size was recovered in a baghouse. (Calbeck U.S. Pat. No. 1,511,215 (1924) and Garesche U.S. Pat. No. 2,065,218 (1936)).